Water-absorbing polymers are especially polymers of (co)polymerized hydrophilic monomers, graft (co)polymers of one or more hydrophilic monomers on a suitable graft base, crosslinked cellulose ethers or starch ethers, crosslinked carboxymethylcellulose, partly crosslinked polyalkylene oxide or natural products swellable in aqueous liquids, for example guar derivatives. Such polymers, as products which absorb aqueous solutions, are used to produce diapers, tampons, sanitary napkins and other hygiene articles, but also as water-retaining agents in market gardening.
Water-absorbing polymers typically have a Centrifuge Retention Capacity of from 25 to 60 g/g, preferably of at least 30 g/g, preferentially of at least 32 g/g, more preferably of at least 34 g/g, most preferably of at least 35 g/g. The Centrifuge Retention Capacity (CRC) is determined according to the EDANA (European Disposables and Nonwovens Association) recommended test method No. 441.2-02 “Centrifuge Retention Capacity”.
The preparation of water-absorbing polymers is described, for example, in “Modern Superabsorbent Polymer Technology”, F. L. Buchholz and A. T. Graham, Wiley-VCH, 1998, pages 69 to 117. Water-absorbing polymer particles are preferably transported by means of pneumatic delivery systems. The mechanical stress which inevitably occurs leads to undesired attrition. Therefore, low transport speeds and hence reduced mechanical stresses are desirable.
In principle, a distinction can be drawn between three different delivery types in pneumatic conveying.                1. In the case of aerial delivery and stream delivery in the region of high gas rates, the laws of the free-flowing individual particle apply approximately. This is the classical type of pneumatic delivery. No product deposits whatsoever occur. There is essentially uniform delivery material distribution in the tube.        2. When the gas rate falls, the delivery moves into the range of strand delivery, where the delivery material flows in the lower half of the tube in particular. In the upper half of the tube, there is aerial delivery.        3. At low gas rates, the delivery proceeds extremely gently as dense stream delivery (plug delivery, impulse delivery) with high pressure drop.        
In principle, the pressure delivery can work with slower delivery rates than suction delivery, since the pressure reserves under elevated pressure are greater than under reduced pressure, and since the delivery gas density which drives the product onward increases with rising pressure.
Since delivery gas is compressible, there is no constant pressure in the delivery line, but rather a higher pressure at the start than at the end. However, this also changes the gas volume, so that, at the start, at higher pressure, slower gas rates predominate, and, at the end, at lower pressure, higher gas rates predominate.
H. Kalman, Powder Technology 104 (1999) 214-220 describes investigations of the attrition in pneumatic delivery systems. Owing to the relatively low mechanical stress, relatively low delivery rates are advantageous. According to the publication, often unnecessarily high delivery rates are, however, often selected for safety reasons in pneumatic delivery.
Excessively low delivery rates in the region of strands delivery are problematic, since stable delivery is not possible in the unstable region between dense stream delivery and strand delivery. Instead, the mechanical stresses which occur can lead to severe damage to the delivery system, up to and including pulling of the delivery lines out of the mounts.
It was an object of the present invention to provide an improved process for pneumatic delivery of water-absorbing polymer particles, reliable operation being possible especially at low delivery rates.